Complications from metastatic disease are the primary cause of death in human breast cancer. Therefore, the long term objective of this project is to understand mechanisms that determine breast cancer metastasis. The central hypothesis is that stable expression of adhesion receptor integrin avbeta3 in a functionally activated state, promotes the metastatic phenotype in breast cancer cells. Integrin alphavbeta3 is here defined as activated, if it supports breast cancer cell interaction with platelets during blood flow, thereby mediating breast cancer cell arrest, and if it enhances breast cancer cell migration. The specific aims are to 1. Analyze whether breast cancer cells expressing activated alphavbeta3 have a selective advantage during tumor growth, and represent the malignant cells that metastasize. This will be tested in a new breast cancer cell model, where cell variants stably express either activated or non-activated alphavbeta3, and are genetically tagged with fluorescent labels. The fate and distribution of these tumor cell variants will be tracked in immune deficient mice; 2. Analyze breast cancer cell functions that are controlled by activated breast cancer cell integrin alphavbeta3, and that contribute to the metastatic activity. The hypothesis to be tested is that activated avbeta3 supports specific ligand binding, adhesive, migratory and invasive breast cancer cell functions, that are required during breast cancer metastasis. This will be tested in vitro, with functional variants of the breast cancer cell model and with primary metastatic cells from breast cancer patients; 3. Clarify whether platelets play a role in breast cancer metastasis. A role of platelets in tumor metastasis has long been discussed, and a prominent function of activated alphavbeta3 in metastatic breast cancer cells is, to support interaction with platelets during blood flow and thereby tumor cell arrest. A contribution of platelets to breast cancer metastasis will be tested definitively in a new mouse model, that lacks circulating platelets and accepts human tumor cell grafts. Together, the proposed studies will contribute to an understanding of specific adhesive mechanisms, that are controlled by breast cancer cell integrin activation, and that support the change from a sedentary to a disseminating metastatic phenotype.